Chip-based advanced modulation format transmitter

ABSTRACT

In various embodiments, a monolithic integrated transmitter, comprising an on-chip laser source and a modulator structure capable of generating advanced modulation format signals based on amplitude and phase modulation are described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/267,582 filed on May 1, 2014 titled “Chip-Based Advanced ModulationFormat Transmitter”, which is a continuation of U.S. application Ser.No. 13/761,867 filed on Feb. 7, 2013 titled “Chip-Based AdvancedModulation Format Transmitter” (now U.S. Pat. No. 8,718,486), which is acontinuation of U.S. application Ser. No. 12/789,350 filed on May 27,2010 titled “Chip-Based Advanced Modulation Format Transmitter,” (nowU.S. Pat. No. 8,401,399), which claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application 61/182,017 filed on May 28, 2009titled “Chip-Based Advanced Modulation Format Transmitter,” and claimsthe benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application61/182,022 filed on May 28, 2009 titled “Monolithic Widely-TunableCoherent Receiver.” Each of the above-identified application isincorporated by reference herein in its entirety.

BACKGROUND

Field of the Invention

Various embodiments of the invention relate to the area of opticalcommunications photonic integrated circuits (PICs). In particular,various embodiments relate to integrated optical transmitters capable ofgenerating multilevel optical modulation formats in opticalcommunications applications.

Description of the Related Art

As demand for higher capacity in optical networks continues to grow,ways to increase optical network capacity with reduced capitalinvestment are of interest. One cost efficient solution that allows forincreased or maximum utilization of the existing optical networkinfrastructure is to implement more spectrally efficient modulationformats for increased data throughput capacity. Advanced modulationformats such as Quadrature Amplitude Modulation (QAM), Phase ShiftKeying (PSK), and Quadrature Phase Shift Keying (QPSK) are spectrallyefficient and can improve the efficiency of fiber Wavelength DivisionMultiplexing (WDM). Modulation formats such as Quadrature Phase ShiftKeying and Quadrature Amplitude Modulation can allow for a number ofdata symbols to be sent utilizing the same line rate as a lower bit-rateOn-Off keyed system. Present optical transmitters for generating opticalsignals having advanced modulation formats are large-scale monolithicphotonic integrated circuits (PICs) or use hybrid platforms. Due totheir size, large-scale PICs may require low waveguide losses, highperformance optical sources and other optical subcomponents. Moreover,refined fabrication processes and techniques may be required to reducedefects and to improve yield of large-scale PICs. Thus, there is a needfor optical transmitters having a reduced footprint and a tolerance foroptical waveguide losses that can be fabricated with simple integrationplatforms.

SUMMARY

Systems and methods that enable an optical transmitter capable ofgenerating optical signals with advanced modulation formats may bebeneficial in optical networks and systems. Example embodimentsdescribed herein have several features, no single one of which isindispensible or solely responsible for their desirable attributes.Without limiting the scope of the claims, some of the advantageousfeatures will now be summarized.

Various embodiments described herein include a compact opticaltransmitter having a reduced die size. For example, the die size of thevarious embodiments of the optical transmitter device described hereincan be between approximately 0.5 square mm and approximately 3 squaremm. In various embodiments, the die size of the optical transmitterdevice can be between approximately 1.5 square mm and approximately 2.5square mm. In various embodiments, the die comprises a monolithicallyintegrated optical transmitter device that is included in packaging toform the device. In various embodiments, the die can comprise amonolithically integrated optical transmitter device that will becoupled to optical fibers or RF/electrical connectors. The decrease inthe footprint and/or the die size of the optical transmitter device canadvantageously reduce fabrication complexity required to integrate asingle surface-ridge waveguide structure and improve yield. Variousembodiments of the optical transmitter described herein can comprise atunable laser resonator and an optical vector modulator. In variousembodiments, an optical vector modulator can include a modulatorarrangement capable of modulating both optical intensity and opticalphase to generate optical vector modulation. Examples of optical vectormodulation formats include QPSK modulation and multilevel QAMmodulation. An arrangement for passive modulator bias control can beimplemented in various embodiments of the optical transmitter to adjustfor the wavelength dependence of the modulator.

In various embodiments described herein, an optical transmittercomprising a widely tunable laser and one or more optical vectormodulators may be monolithically integrated on a single die having acommon substrate. In various embodiments, monolithic common substrateintegration can include processes and techniques that place all thesubcomponents of the device on a common substrate through semiconductordevice processing techniques (e.g. deposition, epitaxial growth, waferbonding, wafer fusion, etc). In some embodiments, the opticaltransmitter comprising a widely tunable laser and one or more opticalvector modulators may be integrated on a single die having a commonsubstrate, through other techniques such as flip-chip bonding, etc.Monolithic common substrate integration can provide a reduction indevice insertion losses. Such tunable optical transmitter devices canallow for a reduction in the number of components and devices requiredin an optical system. Other advantages of an integrated tunable opticaltransmitter can be compact die size, reduced footprint, faster tuningmechanisms, and the lack of moving parts—which can be desirable forapplications subject to shock, vibration or temperature variation.Integrating an optical transmitter on a single die can offer severalother advantages as well, such as precise phase control, improvedperformance and stability of the transmitter, and compactimplementation. Some additional benefits of integrating a tunable laserwith an optical modulator on a single die can be: the ability to adjustor optimize the device performance; ability to control and optimize thebias of the modulators for every single wavelength—(the wavelengthinformation is known for an integrated transmitter, but not known when adiscrete modulator is used); and ability to utilize feedback fromon-chip integrated tap signals in order to better control the operatingpoint of the chip.

Various embodiments, described herein include a complex opticaltransmitter fabricated on a small die size. Such devices can befabricated using relatively simple fabrication techniques and/orintegration platforms. In various embodiments described herein, opticalinterconnect losses can be reduced by reducing interconnect lengthrather than by including complex low-loss optical waveguide structures.

Various embodiments of the optical transmitter described herein comprisea common substrate comprising a III-V material such as Indium Phosphideand one or more epitaxial layers (InP, InGaAs, InGaAsP, InAlGaAs etc.);a laser resonator, formed on the common substrate in the epitaxialstructure; and one or more modulator structures comprising a pluralityof arms or branches and at least two electrodes formed on the commonsubstrate. The one or more modulator structures may be configured tomodulate the amplitude, the phase, or both amplitude and phase ofoptical radiation emitted from the laser resonator. In variousembodiments, the modulator structures may modulate light in accordancewith the principles of optical interference. In some embodiments, themodulator structures may be positioned external to the laser cavity andbe optically connected to the laser resonator. In various embodiments,the various components of the optical transmitter such as waveguides,photonic components, splitters, etc. can be formed in the same epitaxialstructure as the epitaxial structure in which the laser is formed. Insome embodiments the components of the optical transmitter such aswaveguides, photonic components, splitters, etc. can be formed in one ormore epitaxial structures that are different from the epitaxialstructure in which the laser is formed.

In various embodiments a monolithically integrated optical transmitterdie is described. In various embodiments, the size of the monolithicallyintegrated optical transmitter die can be less than approximately 3square mm. The monolithically integrated optical transmitter diecomprises at least one monocrystalline substrate. The monolithicallyintegrated optical transmitter die further comprises a tunable laserresonator monolithically integrated with the substrate, the tunablelaser resonator comprising an output reflector and a tuning section,said tunable laser resonator configured to emit optical radiation fromthe output reflector along an optical axis, such that the wavelength ofthe emitted optical radiation is tunable over a wide wavelength rangefrom between about 15 nm to about 100 nm, wherein the wide wavelengthrange is represented by Δλ/λ and is configured to be greater than aratio Δn/n, wherein λ represents the wavelength of the opticalradiation, Δλ represents the change in the wavelength of the opticalradiation, n represents the refractive index of the tuning section, andΔn represents the change in the refractive index of the tuning section.The monolithically integrated optical transmitter die further comprisesa first optical vector modulator monolithically integrated with thesubstrate, the first optical modulator comprising a first opticalsplitter optically connected to the laser resonator, a plurality of armscomprising at least two electrodes and a first output coupler; and asecond optical vector modulator monolithically integrated with thesubstrate, said second optical modulator comprising a second opticalsplitter optically connected to the laser resonator, a plurality of armscomprising at least two electrodes and a second output coupler. Themonolithically integrated optical transmitter die further comprises apolarization rotator monolithically integrated with substrate, saidpolarization rotator arranged at an angle between about 20 deg and 160deg or between about −20 deg and −160 deg with respect to the opticalaxis; and an optical combiner monolithically integrated with thesubstrate and configured to combine optical signals output from thefirst and second optical couplers. In various embodiments, the firstand/or the second optical splitters can be disposed at a distance ofapproximately less than 750 μm from the output reflector of the laserresonator as measured along the optical axis.

In various embodiments, a method of manufacturing a monolithicallyintegrated optical transmitter is described. The method comprisesproviding at least one monocrystalline substrate. The method furtherincludes monolithically integrating a tunable laser resonator with thesubstrate, the tunable laser resonator comprising an output reflectorand a tuning section, said tunable laser resonator configured to emitoptical radiation from the output reflector along an optical axis, suchthat the wavelength of the emitted optical radiation is tunable over awide wavelength range from between about 15 nm to about 100 nm, whereinthe wide wavelength range is represented by Δλ/λ and is configured to begreater than a ratio Δn/n, wherein λ represents the wavelength of theoptical radiation, Δλ represents the change in the wavelength of theoptical radiation, n represents the refractive index of the tuningsection, and Δn represents the change in the refractive index of thetuning section. The method further includes monolithically integrating afirst optical vector modulator with the substrate, said first opticalmodulator comprising a first optical splitter optically connected to thelaser resonator, a plurality of arms comprising at least two electrodesand a first output coupler. The method can further includemonolithically integrating a second optical vector modulator with thesubstrate, said second optical modulator comprising a second opticalsplitter optically connected to the laser resonator, a plurality of armscomprising at least two electrodes and a second output coupler. Themethod also comprises monolithically integrating a polarization rotatorwith substrate, said polarization rotator arranged at an angle betweenabout 20 deg and 160 deg or between about −20 deg and −160 deg withrespect to the optical axis; and monolithically integrating an opticalcombiner with the substrate, said optical combiner configured to combineoptical signals output from the first and second output couplers. Invarious embodiments, the first and/or the second optical splitters canbe disposed at a distance of approximately less than 750 μm from theoutput reflector of the laser resonator as measured along the opticalaxis. In various embodiments, the monolithically integrated opticaltransmitter can have a die size less than 3 square mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an embodiment of an optical transmitterhaving a reduced footprint and including a laser resonator and a pair ofoptical vector modulators. FIG. 1A schematically illustrates amonolithic tunable laser assembly with folded laser implementation.

FIG. 2 schematically illustrates an embodiment of an optical vectormodulator.

FIG. 3 schematically illustrates an embodiment of an optical vectormodulator bias control.

FIG. 4 schematically illustrates another embodiment of an opticaltransmitter having a reduced footprint and including a laser resonatorand a pair of optical vector modulators. FIG. 4A schematicallyillustrates a monolithic semiconductor polarization multiplexed opticaltransmitter device utilizing a dual output tunable laser assembly.

FIG. 5 schematically illustrates another embodiment of an optical vectormodulator bias control.

FIG. 6 schematically illustrates another embodiment of an opticaltransmitter having a reduced footprint and including a laser resonatorand a pair of optical vector modulators.

FIG. 7 schematically illustrates another embodiment of an opticaltransmitter having a reduced footprint and including two laser resonatorand a pair of optical vector modulators.

FIG. 8A schematically illustrates top view of an integrated of thepolarization rotator.

FIG. 8B schematically illustrates a cross-sectional view of anintegrated of the polarization rotator.

FIG. 8C schematically illustrates an embodiment of a tunablepolarization rotator.

FIG. 9 schematically illustrates another embodiment of an opticaltransmitter.

These and other features will now be described with reference to thedrawings summarized above. The drawings and the associated descriptionsare provided to illustrate embodiments and not to limit the scope of thedisclosure or claims. Throughout the drawings, reference numbers may bereused to indicate correspondence between referenced elements. Inaddition, where applicable, the first one or two digits of a referencenumeral for an element can frequently indicate the figure number inwhich the element first appears.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Although certain preferred embodiments and examples are disclosed below,inventive subject matter extends beyond the specifically disclosedembodiments to other alternative embodiments and/or uses and tomodifications and equivalents thereof. Thus, the scope of the claimsappended hereto is not limited by any of the particular embodimentsdescribed below. For example, in any method or process disclosed herein,the acts or operations of the method or process may be performed in anysuitable sequence and are not necessarily limited to any particulardisclosed sequence. Various operations may be described as multiplediscrete operations in turn, in a manner that may be helpful inunderstanding certain embodiments; however, the order of descriptionshould not be construed to imply that these operations are orderdependent. Additionally, the structures, systems, and/or devicesdescribed herein may be embodied using a variety of techniques includingtechniques that may not be described herein but are known to a personhaving ordinary skill in the art. For purposes of comparing variousembodiments, certain aspects and advantages of these embodiments aredescribed. Not necessarily all such aspects or advantages are achievedby any particular embodiment. Thus, for example, various embodiments maybe carried out in a manner that achieves or optimizes one advantage orgroup of advantages as taught herein without necessarily achieving otheraspects or advantages as may also be taught or suggested herein. It willbe understood that when an element or component is referred to herein asbeing “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent therebetween.

FIG. 1 schematically illustrates an embodiment of an optical transmitterdevice. The device comprises at least one monocrystalline substrate 101,a laser resonator 102, one or more optical vector modulators 106 a and106 b, a polarization rotator 121 and an optical coupler 123. In variousembodiments, the various sub-components of the optical transmitter maybe monolithically integrated with the substrate 101. The optical vectormodulators 106 a may include an optical splitter 107 connected to thelaser resonator 102, a plurality of arms comprising at least twomodulation electrodes and an optical coupler 116. The optical vectormodulator 106 b may be similar structurally similar to optical vectormodulator 106 a. These and other features are further described below.

Monocrystalline Substrate

In various embodiments, the monocrystalline substrate 101 may compriseone or more epitaxial structures. In various embodiments, an epitaxialstructure may be formed by depositing a monocrystalline film on amonocrystalline substrate. In various embodiments, epitaxial films maybe grown from gaseous or liquid precursors. Because the substrate actsas a seed crystal, the deposited film takes on a lattice structure andorientation identical to those of the substrate. In various embodiments,the epitaxial structure comprises InGaAsP/InGaAs or InAlGaAs layers oneither a GaAs or InP substrate grown with techniques such as MOCVD orMolecular Beam Epitaxy (MBE) or with wafer fusion of an active III-Vmaterial to a silicon-on-insulator (SOI) material.

Laser Resonator

As discussed above in various embodiments, the laser resonator 102 maybe formed on the common substrate and/or in one or more epitaxialstructures formed on the common substrate. In various embodiments, theone or more epitaxial structures can include layers or stacks of layersgrown, deposited or formed on the common substrate such that the one ormore layers have a lattice structure and orientation substantiallysimilar to the common substrate. In various embodiments, the laserresonator 102 can include a widely tunable laser. In variousembodiments, the widely tunable laser can comprise a lasing cavitydisposed between two mirrors or reflectors and a tuning section. Theoptical radiation or laser light generated by the widely tunable laseris output from the reflector disposed closer to the output side of thelaser cavity (output reflector) along an optical axis. In variousembodiments of the optical transmitter device the optical axis can bealigned parallel to the crystallographic axis of the monocrystallinesubstrate 101 (e.g. 011 axis for an InP substrate). In the embodimentillustrated in FIG. 1, the optical axis can be aligned parallel to the+y axis.

In various embodiments, the wavelength of the optical radiation emittedfrom the widely tunable laser can be tuned over a wide wavelength rangefrom between about 15 nm to about 100 nm. Without subscribing to anyparticular theory, in various embodiments, the widely tunable laser canhave a relative wavelength change (Δλ/λ) that is larger than theavailable relative index tuning (Δn/n) inside the laser cavity, whereinλ represents the wavelength of the optical radiation, Δλ represents thechange in the wavelength of the optical radiation, n represents therefractive index of the tuning section, and Δn represents the change inthe refractive index of the tuning section. The widely tunable laseroscillator can be configured to tune to any transmission wavelength in agiven range, wherein the range may be larger than the range that can beachieved by refractive index tuning of the semiconductor material and/orthe tuning section alone. Without subscribing to any particular theory,the wide wavelength tuning in some embodiments of the widely tunablelaser can be achieved by using the Vernier effect, in which the twomirrors or reflectors defining the lasing cavity have multiplereflection peaks. The lasing wavelength is then defined by the overlapbetween one reflection peak of each mirror. Tuning the index in one ofthe mirrors or the tuning section (e.g. by applying a voltage toelectrodes disposed on the mirrors and/or the tuning section) can shiftthe wavelength of each of the many reflections, causing a different pairof reflection peaks to come into alignment, thus shifting the lasingwavelength further than that of the wavelength shift of the tunedmirror.

In various embodiments, the widely tunable laser as described herein canhave a tuning range from about 15 nm to about 100 nm around 1550 nm. Insome embodiments, the laser resonator 102 can have a tuning range thatis greater than approximately 15 nm. In certain embodiments, the tuningrange may be approximately 40 nm to 100 nm. In some embodiments, thetuning range may be approximately 20 nm, approximately 25 nm,approximately 30 nm, approximately 35 nm, approximately 40 nm,approximately 45 nm, approximately 50 nm, approximately 55 nm,approximately 60 nm, approximately 65 nm, approximately 70 nm,approximately 75 nm, approximately 80 nm, approximately 85 nm,approximately 90 nm, or approximately 95 nm. In certain embodiments, thetuning range may have a value between any of the values provided above.In some embodiments, the tuning range may be less than approximately 15nm or greater than approximately 100 nm.

In various embodiments, the laser resonator 102 can include any of avariety of widely tunable lasers such as, for example, Sampled GratingDistributed Bragg Reflector (SGDBR) lasers, Superstructure gratingDistributed Bragg reflector, Digital Supermode Distributed BraggReflector (DSDBR), Y-branch or folded tunable laser, etc.

FIG. 1A shows details of an embodiment of a folded tunable laser.Similar to the laser resonator 102, the folded tunable laser can bewidely tunable, and its wavelength tuning range can be wider than whatcan be achieved through an index change, and the modulated signalproperties such as precise amplitude, phase and chirp, are dynamicallycontrolled by the modulator over the wide wavelength tuning range of thelaser.

The folded tunable laser illustrated in FIG. 1A comprises a front mirror151, back mirror 152, a gain section 153 positioned between the frontand the back mirror or incorporated into the mirrors, and a phasesection 154, all of which are situated along a common waveguide 155. Invarious embodiments, the phase section 154 can comprise a short sectionof passive waveguide in which current can be injected to change theindex in order to shift the cavity mode wavelength into alignment withthe peak reflection of the mirrors. The common waveguide 155 can bestraight, or it can utilize total internal reflection mirrors 156, or asystem of bend waveguides in order to fold the laser and make it morecompact. The bend waveguides can be made of one or all of high contrast,deep waveguides, or lower contrast rib or ridge waveguides. Theepitaxial structure and substrate between the different components ofthe folded laser can be different.

By applying an appropriate combination of currents, a light beam isproduced by the folded laser and any frequency of light within thedesigned tuning range can be emitted from the laser. As discussed above,the wavelength of the emitted light is tunable over a wider wavelengthrange than is achievable by index tuning of any one section. The widerwavelength tuning range is represented by Δλ/λ>Δn/n, where λ representsthe wavelength of the light beam, Δλ represents the change (or delta) inthe wavelength of the light beam, n represents the refractive index of awavelength-defining mirror section of the laser, and Δn represents thechange (or delta) in the refractive index of a wavelength-definingmirror section of the laser.

In some embodiments, an optical amplifier section 103 can be integratedat an output side of the tunable laser 102. The optical amplifiersection 103 can amplify the optical radiation emitted from the laserresonator 102 and in some embodiments, the optical amplifier section 103may be used to control the power generated laser light.

Optical Splitters and Fan-Outs

In various embodiments, the optical radiation from the laser resonator102 can be split into two parts using an optical splitter 104. Invarious embodiments, the optical splitter 104 can include withoutlimitation a multimode interference (MMI) splitter. In variousembodiments, the optical splitter 104 can comprise at least one inputwaveguide and at least two output waveguides configured such thatoptical radiation propagating through the at least one input waveguideis split between the at least two output waveguides. In general,integrating a tunable laser with one or more vector modulators on thesame die may require mitigation of light reflection. To this effect, invarious embodiments, optical splitters and optical couplers can compriseN inputs and M outputs that can allow for light evacuation from thevector optical modulators (e.g. by absorption in the substrate) whenthey are in their unbiased or OFF state. In various embodiments, thenumber of inputs N can be 2, 4, etc. while the number of outputs M canbe 2, 3, etc. In various embodiments, the splitter 104 can split thelight either equally or unequally between the at least two outputwaveguides. In some embodiments, the optical power splitting ratiobetween the at least two output waveguide can be tunable.

In various embodiments, a rapid transverse fan-out of the opticalradiation propagating through the at least two output waveguides of thesplitter 104 can be achieved through the use of a plurality of totalinternal reflection (TIR) mirrors 105, each TIR mirror can be configuredto the change the direction of propagation of the optical radiation, forexample, by about 90 degrees. In some embodiments, S-bends or otheroptical waveguide structures may be used to achieve transverse fan-outof the optical radiation propagating through the at least two outputwaveguides of the splitter 104.

In various embodiments, the (TIR) mirrors can also be integrally formedon the substrate 101. In various embodiments, a TIR mirror can comprisea high index-contrast dielectric—semiconductor interface that allowsdiscrete reflection of the optical mode between two waveguides. Onepurpose of these structures can be to change the direction ofpropagation of the optical radiation. In some embodiments, the TIRmirror can comprise at least one reflective facet arranged at an angle θwith respect to the waveguide that is configured to change the directionof propagation of the optical radiation by approximately 90 degrees.Consecutive reflection by two TIR mirrors can allow a 180 degree changein propagation direction of the optical radiation. TIR mirrors can alsoallow a rapid transversal displacement of the optical radiation, thatcan be advantageous to achieve a compact fan-out of input or fan-in ofoutput optical waveguides from the optical splitters and opticalcouplers in contrast to the more commonly used S-bends which require agradual fan-out to maintain low optical loss. In various embodiments,the use of TIR mirrors can enable a reduction in the die size or thefootprint of the device since the input and output waveguides can befanned-out or fanned-in to achieve the desired separation between thevarious sub-components in relatively less space. Furthermore, thelengths of optical waveguides can be shortened in devices using TIRmirrors so as to reduce optical propagation losses. Various embodiments,comprising S-bends to fan-out or fan-in the input and output waveguideswould likely result in an increase in the die size or the footprint ofthe device, since the lengths of the waveguides with S-bends and/or theradius of curvature of the S-bends cannot be reduced beyond a certainminimum length (e.g. in various embodiments, S-bends can exhibitincreased loss if the radius of curvature is less than 50 microns)without increasing waveguide losses or complicating the integrationplatform. Use of TIR mirrors is thus advantageous to realize complexdevices having reduced die size and footprint by using a simpleintegration platform. Nevertheless, there may be embodiments in whichS-bends or other waveguide structures may be more preferable than TIRmirrors to achieve fan-out of input or fan-in of output opticalwaveguides from the optical splitters and optical couplers.

Optical Vector Modulators

Each of the two fanned-out optical signals from the splitter 104 areinput to separate optical vector modulators 106 a and 106 b. Withoutsubscribing to any particular theory, a vector optical modulator caninclude an optical modulator capable of modulating both opticalintensity and optical phase of an input optical radiation to generateoptical vector modulation. Examples of optical vector modulation formatsinclude but are not limited to QPSK modulation and multilevel QAMmodulation.

In various embodiments, each of the optical vector modulators 106 a and106 b may comprise a multi branch structure comprising multiplewaveguides. In some embodiments, the optical vector modulators 106 a and106 b may include nested Mach-Zehnder modulator (MZM). In variousembodiments, the optical vector modulators can be configured to have lowoptical transmission in their unbiased or OFF state (i.e. when no biasvoltages are applied). In some embodiments, this could be accomplishedby varying the width, the length, and/or the optical path length of thewaveguides associated with the optical vector modulators or othermethods of refractive index variation between the branches of theoptical vector modulators.

Bias Control of Optical Vector Modulators

The operation of an optical vector modulator with a widely tunable lasercan place an increased burden on modulator bias control. In simpleoptical modulator configurations (e.g. simple on-off keying (OOK)modulators) bias control can be provided by simple arrangements. Forexample, for electro-absorption modulators (EAM) or Mach-Zehnder typemodulators (MZM) configured to generate a modulated optical signalhaving a simple modulation format (e.g. OOK) bias adjustment can bemade, either directly to the modulator bias electrode (e.g. in an EAM),or via an external modulator phase tuning pad (e.g. in a MZM). The biasadjustment can be conveniently calibrated with wavelength of the opticalinput to the modulator. In contrast, in optical vector modulatorsconfigured to generate optical signals with complex modulation formats,there can be a multitude of modulator bias controls. For example, in adual polarization nested MZM modulator for QPSK generation, there can beup to a total of 15 phase control electrodes that may need to beadjusted based on the wavelength of the input optical radiation. Variousactive bias control schemes have been demonstrated, however, these canbe impractical for a large number of control points. Further, activebias control schemes can lead to a small distortion of the generatedoptical waveform.

An option for bias control in optical vector modulators may be toimplement passive bias control that can be combined with wavelengthcalibration to achieve the required modulator stability. For example, inoptical vector modulators arranged in a Mach-Zehnder type configurationand implemented by using MMI splitters/couplers, the output MMI couplercan include output ports that may not be utilized to form the outputmodulated optical signal. By integrating a photo detector in theun-utilized port and partial absorbers in all the ports of the MMIcoupler, the optical power in all the ports of the MMI coupler can bemonitored. The output of the photo detector and the partial absorberscan provide information regarding the phase and power in variousbranches of the MZ structure. In some embodiments, the partial absorbercan be a contacted passive waveguide section. Most III-V waveguidecompositions can exhibit partial absorption at 0V applied voltage. Bymeasuring the resulting photocurrent, a relative estimate for theoptical power in the optical waveguide can be obtained. From theinformation obtained from the partial absorbers in combination with theabsolute optical power measurement in unused MMI output port photodetector, optical power in all MMI output ports can be estimated at anyoperating wavelength. This information may then be used to providemodulator bias control. For example, in some embodiments, a feedbackcircuit configured to provide an input electrical signal based on theinformation obtained from the partial absorbers and/or the photodetector in the unused MMI port to one or more electrodes (e.g. the biascontrol electrode) of the optical vector modulators.

Operation of Optical Vector Modulators

In the embodiment illustrated in FIG. 1, the input optical signal to theoptical vector modulator 106 a is further split into two parts by anoptical splitter 107. In various embodiments, the optical splitter 107may be similar to the optical splitter 104 described above. In variousembodiments, the optical splitter 107 can be disposed at a distance ofapproximately 750 microns or less from the output reflector of the laserresonator 102 as measured along the optical axis of the laser resonator102. In various embodiments, the optical splitter 107 can be disposed ata distance of approximately 750 microns or less from the outputreflector of the laser resonator 102 as measured along a horizontaldirection parallel to a first edge of the die (e.g. along the y-axis).In various embodiments, the optical splitter 107 can be disposed at adistance of approximately 150 microns-approximately 500 microns from theoutput reflector of the laser resonator 102 as measured along thevertical direction parallel to a second edge of the die (e.g. along thex-axis). In some embodiments, the optical splitter 107 can be disposedat a distance of approximately 250 microns or less from the outputreflector of the laser resonator 102 as measured along the optical axisof the laser resonator 102. In yet other embodiments, the opticalsplitter 107 can be disposed at a distance of approximately 250 micronsor less from the output interface of the amplifier section 103 asmeasured along the optical axis of the laser resonator 102.

Each of the two outputs from the splitter 107 is further split into twoparts by another optical splitter. For example, in the embodimentillustrated in FIG. 1 the optical splitter 108 further splits one of theoutputs of the splitter 107. The two output signals from the splitter108 are fanned-out using TIR mirrors, S-bends or other waveguidestructures. Each of the fanned-out output from splitter 108 is input toa waveguide (e.g. 140 of FIG. 1) that includes a first electrode (e.g.109 of FIG. 1). An electrical signal can be provided to the firstelectrode 109 to modulate the optical radiation propagating through thewaveguide 140. In various embodiments, the electrical signal provided tothe electrode 109 may have a bandwidth in the range of approximately 5GHz to approximately 50 GHz. In various embodiments, a second electrode110 can be disposed on the waveguide 140. Electric current may beprovided to the second electrode 110 to adjust the phase of the opticalradiation propagating through the waveguide 140. In various embodiments,the electrical current provided to the electrode 110 may have a valuebetween about 0 mA and about 15 mA.

An optical coupler 111 (e.g. multimode interference (MMI) coupler,evanescent coupled-mode coupler, reflection coupler, or Y-branchcoupler) unites the two optical waveguides that are disposed at theoutput of the splitter 108 to form one MZM in the dual nested MZMstructure. In various embodiments, the optical coupler 111 can includeat least two input waveguides and at least one output waveguide. Theoptical coupler 111 can be configured to combine optical radiation fromthe at least two input waveguides either equally or unequally and couplethe combined radiation to the at least one output waveguide. In theembodiment illustrated in FIG. 1, the optical coupler 111 includes afirst and a second output waveguide. In various embodiments, a modulatormonitoring arrangement may be formed by a partial absorber 112 disposedon the first output waveguide and a partial absorber 113 disposed on thesecond output waveguide. In various embodiments, an optical terminationdetector (e.g. a photo detector) 114 may be provided to the first outputwaveguide of coupler 111 to absorb radiation propagating in the firstoutput waveguide.

In various embodiments, the partial absorber 113 in second outputwaveguide is followed by an electrode 115. Electric current may beprovided using a current driver to the electrode 115 such that theoptical phase of the optical signal propagating in the second outputwaveguide may be controlled by controlling the amount of currentprovided to the electrode 115. In various embodiments, the electricalcurrent provided to the electrode 115 may have a value between about 0mA and 15 mA. The optical signal propagating in the second outputwaveguide of coupler 111 is combined with the output from the second MZMin the dual nested MZM structure of optical vector modulator 106 a in acoupler (e.g. a 2×2 MMI coupler) 116 having at least 2 input waveguidesand at least one output waveguide. In various embodiments, the coupler116 can couple the combined optical signal into two output waveguidesthat can further include two partial absorbers 117 and 118, atermination photo detector 119 and an electrode 120 to which current canbe provided to control the optical phase.

In various embodiments, the output optical signal from the opticalvector modulator 106 a is reflected using a TIR mirror, and coupled toan integrated polarization rotator 121 that is configured to rotate thepolarization of the optical signal by approximately 90 degrees. In someembodiments, the polarization rotator 121 may be configured to rotatethe polarization of the optical signal by less than or greater than 90degrees. In various embodiments, the polarization rotator 121 may bedisposed at an angle θ between about 20 degrees and 160 degrees orbetween about −20 degrees and −160 degrees with respect to the opticalaxis of the laser resonator. In various embodiments, the polarizationrotator 121 may be disposed at an angle θ between about 20 degrees and160 degrees or between about −20 degrees and −160 degrees with respectto the crystallographic axis of the monocrystalline substrate. Theoutput optical signal from the polarization rotator 121 is thenrecombined with the output from the second optical vector modulator 106b which is propagated in the optical waveguide 122 in the opticalcoupler (e.g. multimode interference (MMI) coupler, evanescentcoupled-mode coupler, reflection coupler, or Y-branch coupler) 123. Invarious embodiments, the features of the optical vector modulator 106 bcan be structurally and functionally similar to the features of theoptical vector modulator 106 a described above. In various embodiments,the output of the second optical vector modulator 106 b can beconfigured to maintain its original input polarization. Two partialabsorbers 124, 125 can be located at the output of the coupler 123 toprovide additional signal monitoring. In some embodiments, one of theoutput waveguides of the coupler 123 can be terminated in a photodetector (126) while signal propagating in the other output waveguide ofthe coupler 123 can be coupled to an external environment through anoutput edge of the optical transmitter device. Facets may be provided atthe output edge of the optical transmitter device to enable opticalconnection with optical fibers, planar waveguides, other devices andsystems. In some embodiments, a mode converter 127 may be disposedcloser to the output edge of the device to improve coupling efficiency.

Some Preferred Embodiments

Some preferred embodiments are described below. It is understood thatthese represent a few possible embodiments out of a range ofcombinations that have some similarities to the embodiment illustratedin FIG. 1 and the sub-components described therein.

FIG. 2 schematic illustrates an embodiment of a single optical vectormodulator similar to the optical vector modulator 106 a and 106 b ofFIG. 1. The optical vector modulator illustrated in FIG. 2 can includeoptical gain regions 201 a and 201 b and/or 202 a and 202 b within themodulator structure. For example, in some embodiments, optical amplifiersections 201 can be provided after the first splitting stage (e.g.splitter 107 of FIG. 1). In certain embodiments, it may be advantageousto provide an amplifying section after the first splitting stage (e.g.splitter 107 of FIG. 1) instead of providing an amplifying section afterthe laser resonator (e.g. amplifying section or region 103 of FIG. 1),since the amplifying section 201 may be able to deliver twice thesaturated output power to the modulator sections. In some embodiments,optical amplifying section or regions 202 may be provided after thesecond splitting stage (e.g. after the optical splitter 108 of FIG. 1).In some embodiments, if the amplifying regions or sections in differentbranches of the optical vector modulator are spaced too closely, thesaturated output power may be reduced due to heating effects. Toeliminate or reduce heating effects, it may be advantageous to rapidlyfan-out the various branches of the optical vector modulator tolaterally separate the various branches. In some embodiments, TIRmirrors can be used to reduce the footprint of a device having such anarrangement.

FIG. 3 schematically illustrates an embodiment of a modulator biascontrol system. The modulator bias control system illustrated in FIG. 3comprises two partial absorbers 301 and 302 that are located at theoutput ports of coupler 303 which is disposed at the output of aMach-Zehnder structure. In various embodiments the coupler 303 can besimilar to the coupler 111 or 116 of FIG. 1. The partial absorbers 301and 302 may be used to detect a photocurrent that is dependent onoptical power of the signal propagating in the waveguide. The biascontrol system can further include an optical termination detector 304that is configured to absorb the radiation propagating in one of theoutput ports or waveguides of the coupler 303. The fractional relationbetween the photocurrent of the partial absorber 301 and the detector304 can then be related to the photocurrent of partial absorber 302 todetermine the optical waveguide power in the other output port orwaveguide 305.

FIG. 4 schematically illustrates a second embodiment of an opticaltransmitter device. Many of the features of the optical transmitterdevice illustrated in FIG. 4 can be similar to the features describedwith reference to FIG. 1. Similar to the embodiment shown in FIG. 1, thedevice illustrated in FIG. 4 can include a single epitaxial structure401, a laser resonator 402, an optional optical amplifier section 403,an optical splitter 404 (e.g. a MMI splitter), a pair of optical vectormodulators 406 a and 406 b and an optical coupler 418 (e.g. a MMIcoupler). The optical splitter 404 and the MMI coupler 418 can beconfigured to split/combine optical radiation into either equal orunequal parts. In the illustrated embodiments, a rapid transversefan-out of the radiation emitted from the two output ports or waveguidesof the optical splitter 404 can be achieved through the use of multipletotal internal reflection (TIR) mirrors (405), S-bends or otherwaveguide structures. In the embodiment illustrated in FIG. 4, theoptical vector modulator 406 a can include an input 1×4 splitter (e.g.1×4 MMI splitter) 407. The four output signals from the 1×4 splitter 407can be fanned out using TIR mirrors (408) and input into four opticalwaveguides. One or more of the four optical waveguides in the opticalvector modulator 406 a can include a first electrode (e.g. electrode409) to which a modulation signal can be provided to generate anoptically modulated signal. In various embodiments, the electrode 409can be structurally and functional similar to the electrode 109 ofFIG. 1. In various embodiments, the optical vector modulator 406 a canfurther include a second electrode 410 in one or more of the fouroptical waveguides. An electric current can be provided to electrode 410to adjust the phase of the optical signal propagating in the one or moreoutput waveguides. In various embodiments, a 4×3 coupler 411 (e.g. a MMIcoupler) can be provided to unite the optical signal propagating in thefour optical waveguides of optical vector modulator 406 a to obtain themodulated optical signal. A modulator monitoring arrangement asdescribed above with reference to FIG. 1 can be provided herein as well.The modulator monitoring arrangement can be formed by partial absorber412 located at the one or more of the output ports of the 4×3 coupler411 and optical termination detectors 413 and 414 that absorb the lightin the two unused output ports or waveguides of the 4×3 coupler 411. Invarious embodiments, the partial absorber 412 in second port or outputwaveguide of the 4×3 coupler 411 can be followed by an electrode 415 towhich an electric current can be provided to adjust the phase of theoptical signal propagating in the waveguide. The output of the opticalvector modulator 406 a can be reflected using a TIR mirror, S-bend orother waveguide structures, and coupled to an integrated polarizationrotator 416 that rotates the polarization of the optical signal byapproximately 90 degrees. The polarization rotator 416 can bestructurally and functionally similar to the polarization rotator 121 ofFIG. 1. This polarization rotated optical signal can be then combined ina 2×2 coupler 418 (e.g. a MMI coupler) with the output from the secondintegrated optical vector modulator 406 b that is propagated through theoptical waveguide 417. In various embodiments, the optical signalpropagating in the optical waveguide 417 can maintain its original inputpolarization. Two partial absorbers 419 a and 419 b can be located atthe output of the coupler 418 to provide additional signal monitoringcapability. In some embodiments, one of the output waveguides of thecoupler 418 can be terminated in a photo detector 420, while signalpropagating in the other output waveguide of the coupler 418 can becoupled to an external environment through an output edge of the opticaltransmitter device. Facets may be provided at the output edge of theoptical transmitter device to enable optical connection with otherdevices and systems. In some embodiments, a mode converter 421 may bedisposed closer to the output edge of the device to improve couplingefficiency.

FIG. 4A shows a polarization multiplexed transmitter realized using adual output laser source 450, instead of the laser resonator 402 and thelight splitter 404 of FIG. 4. In this embodiment, each of the laseroutputs is connected to a respective one of the optical vectormodulators 406 a and 406 b.

FIG. 5 schematically illustrates an embodiment of a modulator biascontrol system that can be implemented with the optical vector modulatorillustrated in FIG. 4. The modulator bias control illustrated in FIG. 5can include partial absorber 501 a, 501 b, 503 and photo detectors 502 aand 502 b As described above with reference to FIG. 3, the outputwaveguide power can again be estimated by comparing the fractionalrelation of the photocurrent from the partial absorbers 501 a and 501 band the detectors 502 a and 502 b which can then be related to thephotocurrent of middle partial absorber 503 to determine the opticalwaveguide power in the output waveguide 504. Additional modulator biasinformation can be obtained by measuring the balance between the twodetector currents.

FIG. 6 shows a variation of the embodiments illustrated in FIG. 1 orFIG. 4. The illustrated device can comprise a single epitaxial structure601, a laser resonator 602, an optional optical amplifier section 603and a splitter 604 (e.g. a multimode interference (MMI) splitter)splitting the light in two parts. As described above, TIR mirrors 605can be used to rapidly fan out the two output waveguides or ports of thesplitter 604. In the illustrated embodiment, the TIR mirrors areoriented such that the direction of propagation of the optical radiationemitted from the laser resonator 602 is turned by about 180 degreesbefore being input to the optical vector modulators 606 a and 606 b.

FIG. 7 shows another variation of the embodiments illustrated in FIG. 1or FIG. 4. The device illustrated in FIG. 7 comprises a single epitaxialstructure 701, dual laser resonators 702 a and 702 b, optional dualoptical amplifier sections 703 a and 703 b, and a 2×2 splitter 704 (e.g.a 2×2 multimode interference (MMI) splitter) for splitting the light intwo parts. As described above, two optical vector modulators 705 a and705 b can be monolithically integrated with the substrate to generate amodulated optical signal. Such a device may be advantageously used inoptical switching applications, where a rapid transition from oneoptical wavelength to a second optical wavelength may be required. Invarious embodiments, the lasers 702 a and 702 b may be both tuned totheir designated optical wavelengths, while the wavelength switchingevent takes place by turning one amplifier (e.g. 703 a) on while turningthe second amplifier (e.g. 703 b) off, to switch which laser outputlight enters the 2×2 splitter 704.

In various embodiments, polarization beam splitters and rotators can beintegrated on the common substrate in the same or different epitaxialstructure as other sub-components of the integrated optical transmitter.In various embodiments, the polarization beam splitter elements may beformed on the common substrate, in the same or different epitaxialstructure as other components, with the purpose of splitting the inputcoupled light into the TE and TM polarized modes of the commonwaveguide. One embodiment of this element can be realized by using thephenomenon of birefringence between two modes. A polarization rotatingelement can convert TM polarized light into TE polarized light and viceversa.

A polarization rotator can be formed by inducing a birefringence in thewaveguide, for example, by fabricating an asymmetric waveguide. Anasymmetric waveguide can be fabricated by etching the sidewall of theoptical waveguide at an angle. By selecting the appropriate length ofthe angled etch, a halfwave plate can be formed that can rotate linearlypolarized TE or TM light by 90 degrees. Other approaches and designs canalso be used.

FIG. 8A schematically illustrates top view of an embodiment of apolarization rotator that is integrated in the optical transmitterdevice. FIG. 8B schematically illustrates a cross-sectional view of thepolarization rotator illustrated in FIG. 8A along an axis 804 parallelto the normal to the substrate 803 (e.g. parallel to the z-axis). In oneembodiment, the polarization rotator comprises an asymmetric waveguideridge 802 that is disposed on a substrate 803. In various embodiments,the polarization rotator can be formed by modifying a waveguide section801 of the optical transmitter device using semiconductor deviceprocessing techniques. In various embodiments, the asymmetric waveguideridge 802 can be formed on a slab waveguide 805 which comprises ahigh-index material. In some embodiments, the waveguide ridge 802 canhave a first edge 802 a disposed at a first angle with respect to thenormal to the substrate and a second edge 802 b disposed at a secondangle with respect to the normal to the substrate. In variousembodiments, the first and the second angle can be different from eachother. In various embodiments, the first angle can be approximatelyparallel to the normal to the substrate as shown in FIG. 8B. Theasymmetric nature of waveguide ridge 802 results in a bi-refringentwaveguide structure.

The asymmetric waveguide structure 802 can be formed by using an etchingprocess. For example, in one method of fabricating the polarizationrotator, the asymmetric waveguide structure 802 is dry etched on oneside of the waveguide ridge 802 to form the edge 802 a, and wet etchedon the other side of the waveguide ridge 802 to form the sloping edge802 b. In some embodiments, the method can include etching through theslab waveguide 805. Etching through the slab waveguide 805 can beadvantageous to realize a polarization rotator structure with reducedfootprint.

In one method of fabricating the polarization rotator on an InPsubstrate, the sloping edge 802 b can be formed by employing a wet etchat a waveguide section oriented around 90 degrees with respect to thelaser ridge—which gives around 40-50 degrees wet etch plane—that stopson the InGaAsP or InAlGaAs waveguide core (e.g. slab waveguide 805) or astop etch layer. If a different orientation is chosen for thepolarization rotator (e.g. perpendicular to or within 20-160 degrees or−20 to −160 degrees from the laser axis) the wet etch will align thewaveguide edge to an angle not perpendicular to the substrate. The abovedescribed method of fabricating the polarization rotator can be arepeatable process and can yield polarization rotators with a smallfootprint.

FIG. 8C illustrates an embodiment of a tunable polarization rotator. Thetunable polarization rotator comprises an input waveguide 810 that isconnected to an optical splitter 812 having with two output waveguides.In various embodiments, the optical splitter 812 can be a polarizationbeam splitter. Electrodes 814 a, 814 b, 816 a and 816 b may be providedto the two output waveguides. A voltage between approximately 1V toapproximately −6V can be applied to the electrodes 814 a and/or 814 b tochange the transmitted optical intensity. An electric current in therange of approximately, 0 mA to approximately 15 mA may be provided tothe electrodes 816 a and/or 816 b to adjust the phase of the opticalradiation propagating through the output waveguides. In variousembodiments, the voltage and the current can be provided by using anexternal drive circuit. The tunable polarization rotator can furthercomprise a polarization rotator 818 that can be disposed in one of theoutput waveguides. The polarization rotator 818 can be similar to thevarious embodiments of the polarization rotators described above. Thetunable polarization rotator can further comprise an optical coupler 820having an output waveguide 822 and configured to combine the opticaloutputs of the two output waveguides of splitter 812. With theappropriate adjustment of optical phase and intensity, the polarizationstate of the signal in the output waveguide 822 can be tuned.

FIG. 9 shows another embodiment of the optical transmitter device. Thedevice illustrated in FIG. 9 can comprise at least one monocrystallinesubstrate having a single epitaxial structure 901, a laser resonator902, an optional optical amplifier section 903, a 2×2 splitter 904 (e.g.a multimode interference (MMI) splitter) for splitting the light in twoparts. In this embodiment, a second stage optical splitter 905 can beprovided to further split the optical power. The illustrated embodimentmay include four optical vector modulators 906 a, 906 b, 906 c, 906 d.The waveguide at the output of each optical vector modulator 906 a, 906b, 906 c, 906 d can include a pair of electrodes 907 and 908. Anelectrical field can be provided to electrode 907 to control theamplitude of the optical signal in the output waveguide while anelectrical current can be applied to electrode 908 to control theoptical phase of the optical signal propagating in the output waveguide.By combining the output from two optical vector modulators with thecorrect phase and amplitude, more complex optical modulation formatssuch as 16-QAM modulation can be generated in a practical manner. Thisnested arrangement can be expanded for practical generation of evenhigher order modulation formats such as 64 or 128-QAM.

In various embodiments, the various integrated optical transmitterarchitectures and components can be monolithically integrated on acommon substrate with the various integrated receiver architectures andcomponents such as those described in U.S. Provisional App. No.61/182,022 filed on May 28, 2009 titled “Monolithic Widely-TunableCoherent Receiver,” which is incorporated herein by reference in itsentirety, to obtain a single transceiver device.

While the foregoing detailed description discloses several embodimentsof the present invention, it should be understood that this disclosureis illustrative only and is not limiting of the present invention. Itshould be appreciated that the specific configurations and operationsdisclosed can differ from those described above, and that the apparatusand methods described herein can be used in contexts. Additionally,components can be added, removed, and/or rearranged. Additionally,processing steps may be added, removed, or reordered. A wide variety ofdesigns and approaches are possible.

The examples described above are merely exemplary and those skilled inthe art may now make numerous uses of, and departures from, theabove-described examples without departing from the inventive conceptsdisclosed herein. Various modifications to these examples may be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other examples, without departing from thespirit or scope of the novel aspects described herein. Thus, the scopeof the disclosure is not intended to be limited to the examples shownherein but is to be accorded the widest scope consistent with theprinciples and novel features disclosed herein. The word “exemplary” isused exclusively herein to mean “serving as an example, instance, orillustration.” Any example described herein as “exemplary” is notnecessarily to be construed as preferred or advantageous over otherexamples.

What is claimed is:
 1. An optical transmitter comprising: an opticalsource comprising: a waveguide having a first end including a firstreflector and a second end including a second reflector, each of thefirst and the second reflector optically connected to an active regionsuch that light is output from the first and second ends; a firstmodulator comprising a first input waveguide optically connected to thefirst end and configured to receive light output from the first end ofthe optical source, the first modulator configured to modulate at leastone of intensity or phase of light received from the first end; and asecond modulator comprising a second input waveguide optically connectedto the second end configured to receive light output from the second endof the optical source, the second modulator configured to modulate atleast one of intensity or phase of light received from the second end.2. The optical transmitter of claim 1, wherein the optical sourcefurther comprises a passive section including an electrode, whereinindex of refraction of the waveguide is changed by providing a currentto the passive section via the electrode.
 3. The optical transmitter ofclaim 1, wherein the optical source further comprises an opticalredirector configured to change the direction of propagation of opticalradiation in the waveguide by an angle.
 4. The optical transmitter ofclaim 3, wherein the angle is between about 90 degrees and about 180degrees.
 5. The optical transmitter of claim 3, wherein the opticalredirector comprises a reflective facet that is arranged at an anglewith respect to a portion of the waveguide.
 6. The optical transmitterof claim 3, wherein the optical redirector comprises a dielectric. 7.The optical transmitter of claim 1, wherein the waveguide comprises oneor more curved sections.
 8. The optical transmitter of claim 7, whereinthe one or more curved sections include at least one of a deep waveguidesection, a ridge waveguide section or a rib waveguide section.
 9. Theoptical transmitter of claim 1, further comprising an optical combinerconfigured to combine optical signals output from the first modulatorand the second modulator and generate a modulated optical signal. 10.The optical transmitter of claim 9, wherein the optical combinerincludes a third input waveguide connected to the first modulator, afourth input waveguide connected to the second modulator and at leastone optical redirector integrated with the third or fourth inputwaveguide, the optical redirector configured to change the direction ofpropagation of light in the waveguide it is integrated with.
 11. Theoptical transmitter of claim 9, further comprising a polarizationrotator disposed in an optical path between the first or secondmodulator and the optical combiner, the modulated optical signal fromthe first or second modulator being coupled into the polarizationrotator.
 12. The optical transmitter of claim 1, wherein the opticalsource, the first modulator and the second modulator are monolithicallyintegrated on a substrate.
 13. The optical transmitter of claim 12,wherein substrate comprises at least one of Si, InP, InAlGaAs, InGaAsP,InGaP, GaAs or InGaAs.